The present invention relates to a thermal management system for an aircraft gas turbine engine, and more particularly to a heat exchanger cooling duct system that reduces system pressure drop and resists re-ingestion of heated airflow from the heat exchanger during aircraft ground operations.
Aircraft thermal management systems are utilized to cool various components and systems including for example the engine oil system, avionics systems and mechanical components. Tactical aircraft in particular place a premium on space such that systems that require cooling airflow are often packaged within the airframe in a manner which complicates effective thermal management thereof. Such aircraft are required to operate in airport environments which further complicate effective thermal management such as high altitude and hot ambient temperature environments. Still other operational conditions such as where a multitude of aircraft are stacked up on taxiways awaiting take-off in the mid-day sun still further complicates effective thermal management by raising the effective ambient temperature at a time when the aircraft can not be cooled by ingesting air provided by a ram scoop and the vehicle's forward velocity.
Aircraft thermal management systems often include a duct system through which a portion of the air stream is diverted to communicate airflow (e.g. ram air) over a fuel-air heat exchanger. During various operations such as ground operations, this ram air may be supplanted by a fan system which communicates airflow over the heat exchanger. Conventional ducting and door arrangements often result in leakage and pressure losses in either or both operational conditions.
Ground idle operations are one of the most extreme thermal management “corner point” conditions encountered by aircraft. Taxiways are typically in the opposite direction of into-the-wind runways such that tailwinds bearing the hot engine exhaust tend to dominate ground operations. Various conventional thermal management systems often closely locate an exhaust aft of an intake on an aircraft upper surface. This intake and exhaust arrangement may result in the fan system re-ingesting heated exhaust air at close to 100% of the system's discharge temperature. Thermal management may then be further complicated by the re-ingestion of exhausted air from the heat exchanger itself which has been further heated by solar loads as the air flows through the boundary layer on the aircraft upper surfaces. In the case of fuel-air heat exchangers, still other operational conditions, such as low fuel tank levels, may further complicate thermal management because the fuel in the tank starts to heat up as warm, poorly cooled fuel is returned to the tank and mixes in with the balance of the fuel. Various combinations of these adverse conditions may cause fuel tank temperatures to rise relatively quickly and surpass a limiting temperature such as the fuel tank seal allowable limits in the case of fuel systems, and bearing compartment lubrication temperature limits in the case of oil systems, and chemical degradation limits in the case of hydraulic systems.
Accordingly, it is desirable to provide a thermal management cooling duct system which minimizes leakage and pressure losses yet enhances cooling airflow by resistance to self-re-ingestion of heated airflow from the heat exchanger during aircraft ground-idle operations, as well as reducing the cooling system's thermal vulnerability to solar heating.